Autism has been shown to be highly heritable, with heritability estimated at over 90%. Estimates for the number of genes underlying autism range from 3 to over 15 and there is evidence for genetic heterogeneity. Although there are several genome-wide linkage analysis studies that have been completed, there is a disappointing concordance in linked regions between the studies. Chromosomal regions where there is more agreement include 7q and 2q. The evidence for linkage to chromosome 2 was observed in prior studies of ours as well as in other studies in the field. The current application represents a focused attempt to identify an autism susceptibility gene on 2q. Our preliminary data also shows evidence for linkage on chromosome 1 and other sites when the population is stratified based on repetitive behaviors. This study aims to identify an additional 150 families with at least two family members affected with autism, to genotype samples and analyze the data for linkage to autism and repetitive behaviors. In addition, we will look for overlap between areas of linkage for repetitive behaviors and candidate genes of the serotonin system.